An ophthalmic progressive addition lens, or PAL, has an optical power that varies progressively along a line over the surface of this lens, known as meridian line. The meridian line connects a distance-vision point on the lens, to which the optical power and astigmatism of the lens are adapted in order to correct the distance vision of a wearer, to a near-vision point to which the optical power is adapted in order to correct the near vision of the wearer.
A method for vision correction traditionally includes measuring optical aberration data of a wearer's eye, usually performed by optometrists or ophthalmologists, determining the lens parameters and offering to the wearer a plurality of model lenses with different lens “designs”.
The wearer's vision parameters are measured using for example trial lenses, an aberrometer, a wave-front sensor, grating or other known method and/or apparatus.
Other vision parameters can be obtained such as the wearer's vertex distance, pupil size, pupil distance, frame information, gaze directions, eye convergence.
The optical distribution of aberrations or optical “design” imparts the optical correction of the material. Given the infinite number of eyesight corrections, the numbers of designs is almost infinite.
For cost and manufacture reasons, only a limited number of “model designs”, applied on the front surface of the lens is predetermined by the lens manufacturers.
Such “model designs” are of great importance when considering progressive addition lenses (PAL).
PAL have gained worldwide acceptance as the most performant ophthalmic lenses for correction of presbyopia because they provide comfortable vision at all distances.
A PAL is designed not only to restore a presbyope's ability to see clearly at all distances but also to optimally respect all physiological visual functions, in particular:                foveal vision where coordination of the body, head and eye movements, in relation to the objects' location in the vision, defines the power value needed at each point of the progression. The field of gaze is determined by the natural coordination of horizontal eye and head movements;        extra-foveal vision (provided by the periphery of the retina) which provides space and form perception and is directly influenced by the distribution of prism on the progressive addition lens surface. The variation of prismatic effects plays also a role in the wearer's comfort when movement is perceived;        binocular vision where, for optimal fusion of the simultaneous perception of the two eyes, the images produced by the right and left lenses must be formed on corresponding retinal points and display similar optical properties in all directions of gaze.        
Progressive addition lens designers work towards respecting these physiological functions and propose a limited number of optimized designs which are tested through rigorous clinical trials. A plurality of “model designs” is offered by each lens maker.
The optometrist or ophthalmologist proposes usually a lens “model design” that may be the result of an analysis of the viewing behavior of the wearer.
According to a known embodiment, an ophthalmic lens which is adapted to the vision of a wearer is obtained starting from a semi-finished lens with the chosen “model design” which is manufactured in large volume, and which has a finished front face. In other words, the front face of the semi-finished lens has local values of average sphere and of cylinder that vary between different points of this face. The rear face, also called back face, of the semi-finished lens is used to adapt the ophthalmic correction to the ametropia of the wearer. For this purpose, it is re-machined according to a prescription established for the wearer. The rear face may comprise a progressive addition surface so as the lens is called a “dual add” PAL.
The semi-finished lens is selected from amongst several standard models (based on the “model design”) as a function of a curvature value of the front face at the distance-vision point and from an addition value. Depending on these values, the front face of a progressive semi-finished lens has fixed design features values, which are also called standard designs features values. They are determined for average conditions of use of the lens and are fixed when the semi-finished lens is molded.
The principal standard design features are selected in the list consisting of the standard size parameters of the different vision zones of the progressive addition lens (such as for example the near vision zone, the intermediate vision zone, the distance vision zone), standard inset of a wearer, standard frame design parameters, standard viewing preferences.
In a standard progressive addition lens the size of the distance vision zone is greater than the size of the near vision zone, which is greater than the size of the intermediate zone.
In a progressive addition lens, the near-vision point can be shifted horizontally with respect to a vertical line passing through the distance-vision point, when the lens is in a position of use by its wearer. This shift, which is in the direction of the nasal side of the lens, is usually referred to as “inset”. It depends on the optical power of the lens, on the distance of observation of an object, on the prismatic deviation of the lens and on the eye-lens distance, notably. FIG. 1a indicates the positions of the distance-vision and near-vision points of an ophthalmic lens 100, respectively denoted VL and VP, the mounting cross, denoted CM, the meridian line, denoted LM, and the inset, denoted In. FIG. 1b is a profile view of the lens 100, showing the front face of the latter, which is convex and referenced S0, and its concave rear face S1.
Now, it is also known for some design features of a progressive addition lens to be adapted according to the wearer for whom the lens is designed, in particular in order to reduce the time that could be required for the wearer to become accustomed to this progressive addition lens. Such an adaptation of the lens is referred to as ‘customization’ of the design feature.
As for an example where the design feature parameter is the inset, this can be achieved by simply rotating the semi-finished lens about its optical axis, before the rear face has been adjusted to the prescription and before the lens is cut to the dimensions of a frame of a pair of glasses. The inset value can thus be customized, by means of the angle of rotation, as a function of the measurements made on the wearer. But then all the characteristics of the lens, including the variations in astigmatism outside of the distance-vision and near-vision regions, are simultaneously rotated. This results notably in a reduction in the width of the distance-vision region, measured in a horizontal direction, which may be detrimental to the comfort of the wearer.
There is thus, now a trend to customize progressive addition lenses to the wearer's eyes specificities, in particular to customize the inset.
The inset is usually calculated according to the average sphere in the near vision zone and the reading distance of the wearer.
Although these parameters may give a rough approximation of the inset, there is a need to improve the calculated value of the inset.
One goal of the present invention therefore consists in providing a method for adjusting the value of the inset of a progressive addition lens according to a wearer and that is more accurate than the known methods.